1. Field of Invention
The present invention is in the field of battery technology and, more specifically, relates to charging profiles for batteries used in inverter applications such as in conjunction with uninterruptable power supply (UPS) systems, or other applications that involve sporadic charging of batteries.
2. Discussion of Related Art
The use of uninterruptible power supplies to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems and other data processing systems, is known. A number of different UPS products are available including those identified under the trade name Smart-UPS® from American Power Conversion Corporation (APC) of West Kingston, R.I. In a typical UPS, a battery is used to provide backup power for a critical load during blackout or brownout conditions. Some examples of UPS systems include flooded cell lead-acid batteries that are used to provide back-up power. It has been observed that in certain environments, flooded cell lead-acid batteries may provide reduced backup time and have shortened life span due to charge starvation, sulphation of the electrodes, and/or electrolyte loss when charged using conventional charge profiles, such as those commonly recommended by battery manufacturers.
One example of a conventional charge profile utilized by some battery chargers for charging flooded cell lead-acid batteries is illustrated in FIG. 1. The charge profile of FIG. 1 includes three regions of operation, as is typically recommended by battery manufacturers for flooded cell lead-acid batteries. In the first region, termed the constant current region (CC region 10), the battery charging current is substantially constant. In the CC region 10, the voltage across the terminals of the battery increases as charge is added to the battery. After the battery voltage reaches a certain level (VBOOST) at time 15, the profile changes to a second region of operation in which the voltage applied across the battery terminals is held constant. This region of operation is termed the constant voltage region (CV region). The CV region is divided in to two parts, namely, a boost region 20 and a float region 30. In the boost region 20, the charging voltage is maintained at a higher level than the open circuit voltage of the battery. After a predetermined amount of time in the boost region 20, terminating at time 25, the voltage applied across the terminals of the battery is reduced to a second level (VFLOAT) and the charge profile enters the float region 30. The charger maintains the battery in a constant voltage charge mode at the VFLOAT voltage level until the battery is needed to provide back-up power.
There is a second type of conventional charging profile (not illustrated), in which the charging profile stays in the CC region until the battery voltage just touches the boost region voltage level, at which point the voltage applied by the charger is reduced to the float region voltage. Charging at the float region voltage may cause limited evaporation of electrolyte, but the charging is slow. Accordingly, it may take an unacceptably long time to attain a desired state of charge and adequate de-sulphation of the battery electrodes.